For most mammals, chemical communication is an important source of information about others of the same species. Even in humans, the well-documented menstrual synchrony among women living together is evidence for subliminal communication based on odor cues. Numerous less well-documented anecdotes suggest that chemical communication may also carry other information between individuals. In many mammals, the main olfactory system and an accessory system, the vomeronasal organ and its associated central neural pathways, act together and have a large influence on behavior. The presence of functional vomeronasal organs in humans is questionable but the brain structures evolved to process vomeronasal information, the medial and cortico-medial amygdala, are still prominent in the human brain and still receive chemosensory input. This proposal describes experiments to investigate the function of these brain regions. The model system for these studies is the chemical communication that leads to male hamster reproductive and territorial behavior. Activation of the central vomeronasal neural pathway and the regions it connects to, are the most obvious events in the forebrain during mating or stimulation with female chemosignals - as assessed by the expression of the rapidly induced gene c-fos. Chemosensory stimuli from females and territorial chemical-signals from other males strongly activate this pathway in male hamsters, including both the anterior and posterior medial amygdala. Preliminary data suggest that chemosensory stimuli from other species, biologically important but with no "social" relevance for hamsters strongly activate the first stage of one branch of the pathway (anterior medial amygdala), but that activation is not passed on to the next brain region (posterior medial amygdala). Artificial stimulation of the vomeronasal pathway by electrical stimulation or an excitatory drug also activates anterior but not posterior medial amygdala. We will test the hypothesis that there is a functional filter in the amygdala that selects input information for onward transmission and processing by posterior medial amygdala on the basis of its social relevance. The accessory olfactory bulb, a prior relay in the system, appears differently selectivity for the same stimuli. We will use electrophysiology and c-los gene expression to confirm these selectivities of response and to investigate whether spatial and temporal patterns of activation, or the activation of certain classes of cells, could be the mechanism.